There are numerous physical measurements that attempt to deduce a property of a sample by observing the light reflected from the sample when the sample is illuminated with light. For example, information about the chemical composition of a sample can often be deduced by observing the fluorescent spectrum induced by illuminating the specimen with light of a known wavelength.
To make such measurements, light must be conducted to the specimen and the light emitted therefrom collected for analysis. Single and multimode optical fibers are often used to deliver the light to the specimen. Such fibers are easily manipulated and have high transmission over a considerable bandwidth. In principle, the same fiber can be used to collect the light leaving the specimen. While this type of collection scheme provides an attractive solution to the light collection problem, it functions poorly in those cases in which the position and/or tilt of the specimen changes during the measurement process.
A number of measurement applications are designed to measure the properties of a moving specimen. For example, interferometric techniques can be utilized to measure the thickness and composition of thin films as the films are manufactured. Unfortunately, the films are moving at relatively high speeds and tend to "flutter". The flutter results in changes in angle and/or distance of the film relative to the optical fiber used to deliver light to the film and collect the light reflected back by the front and back surfaces of the film.
Typically, a lens is used to couple the light between the optical fiber and the surface. There are two common lens configurations, collimating and imaging. In a collimating configuration, the light leaving the fiber is expanded into a beam of parallel rays having a diameter much larger than the optical fiber core. Light that returns parallel to the direction of the original ray bundle will be focused by the lens back into the optical fiber. If the tilt of the surface changes, this condition will not be satisfied, and hence, the collection efficiency will be poor. The distance between the lens and the surface has only a weak effect on the collection efficiency. Hence, the collimating configuration is insensitive to movement of the surface along the beam directions.
In the imaging configuration, the lens is used to form an image of the fiber core on the specimen. The specimen is thus illuminated at a point. This configuration is relatively insensitive to tilting of the specimen. However, changes in the distance between the lens and the specimen result in the illuminated spot on the specimen expanding. Since imaging of the illuminated spot back into the fiber depends upon the correct spacing between the lens and the specimen, the efficiency of light collection in the imaging configuration is very sensitive to changes in the distance between the lens and the specimen.
Broadly, it is the object of the present invention to provide an improved alignment system for optical coupling lens system used in coupling light from an optical fiber to a specimen.
It is a further object of the present invention to provide an alignment system that can correct for changes in orientation of the specimen surface.
It is a still further object of the present invention to provide an alignment system that can correct for changes in distance between the lens system and the specimen surface.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.